Alzheimer's disease (AD) is a late onset neurodegenerative disorder characterized by the extracellular deposition of insoluble aggregates composed of the 40 to 42 amino acid A.beta. peptide in the brain (Glenner and Wong, Biochem. Biophys. Res. Commun. 120:885-890 (1984); Masters et al., EMBO J. 4:2757-2763 (1985)). A.beta. peptide is derived from an integral membrane protein termed amyloid .beta.-protein precursor protein (APP) (Tanzi et al., Science 235:880 (1987); Kang et al., Nature 325:733-736 (1987)). The function and metabolism of APP have been the subject of intensive study due to the fact that mutations in APP are associated with an autosomal dominant form of AD, (Goate et al., Nature 349:704-707 (1991)) and over-production of APP is the presumptive cause of AD in trisomy 21 (Tanzi et al., Science 235:880 (1987); Hyman et al., Proc. Natl. Acad. Sci. USA 92:3586-3590 (1995)). Multiple APP isoforms can be generated by alternatively splicing of mRNAs. The major isoforms in brain are APP695, APP751, and APP770 containing 695, 751 and 770 amino acids, respectively. These isoforms are transmembranous proteins having large extracellular regions, with hydrophobic membrane spanning domains and short cytoplasmic segments. APP is also a member of an evolutionary conserved family of proteins which include the APP-like proteins, APLP1 and APLP2 (Wasco et al., Proc. Natl. Acad. Sci. USA 89:10758-10762 (1992); Wasco et al., Nature Genet. 5:95-100 (1993); Slunt et al., J. Biol. Chem. 269:2637-2644 (1994)).
Secreted forms of APP are generated by proteolytic cleavages within their extracellular domain close to the transmembrane region. The extracellular regions of APP751, APP770, and APLP2 each contain a Kunitz protease inhibitor (KPI) domain encoded by an alternatively-transcribed exon (Kitaguchi et al,Nature 331:530-532 (1988); Tanzi et al., Nature 331:528-530 (1988); Wasco et al, Nature Genet. 5:95-100 (1993); Slunt et al., J. Biol. Chem. 269:2637-2644 (1994)). Secreted forms of APP having the KPI domain correspond to a protease inhibitor that has been identified separately and named protease nexin II (APP/PN-2) (Van Nostrand and Cunningham, J. Biol. Chem. 262:8508-8514 (1987); Oltersdorf et al., Nature 341:144-147 (1989); Van Nostrand et al., Nature 341:546-549 (1989)), a potent inhibitor of the blood coagulation factors IXa (Schmaier et al., J. Clin. Invest. 92:2540-2545 (1993)) and XIa (Van Nostrand et al., J. Biol. Chem. 265:9591-9594 (1990)). APP/PN-2 binds with high affinity to cultured fibroblasts (Johnson-Wood et al., Biochem. Biophys. Res. Commun. 200:1685-1692 (1994)), and APP/PN-2:proteinase complexes are internalized and degraded by cultured cells (Knauer and Cunningham, Proc. Natl. Acad. Sci. USA 79:2310-2314 (1982); Knauer et al., J. Cell. Physiol. 117:385-396 (1983)) although the mechanism for this process is unknown. Recent studies have identified the low density lipoprotein receptor-related protein (LRP) as the receptor responsible for the catabolism of another Kunitz-type inhibitor, tissue factor pathway inhibitor (TFPI) (Warshawsky et al., Proc. Natl. Acad. Sci. USA 91:6664-6668 (1994)).
LRP is a large multiligand receptor (Krieger and Herz, Annu. Rev. Biochem. 63:601-637 (1994)) that is a member of the LDL receptor family, which also includes the LDL receptor (Yamamoto et al., Cell 39:27-38 (1984)), the VLDL receptor (Takahashi et al., Proc. Natl. Acad Sci. USA 89:9252-9256 (1992)), and glycoprotein 330 (Saito et al., Proc. Natl. Acad. Sci. USA 91:9725-9729 (1994)). A 39 kDa protein, termed the receptor associated protein (RAP) (Strickland et al., J. Biol. Chem. 266:13364-13369 (1991)) binds to members of the LDL receptor family (Williams et al., J. Biol. Chem. 267:9035-9040 (1992); Kounnas et al., J. Biol. Chem. 267:21162-21166 (1992); Battey et al., J. Biol. Chem. 269:23268-23273 (1994)) and blocks their ligand binding capacity. LRP mediates the cellular uptake and subsequent degradation of proteinases, such as tissue-type plasminogen activator (Bu et al., Proc. Natl. Acad Sci. USA 89:7427-7431 (1992)) and urokinase-type plasminogen activator (Kounnas et al., J. Biol. Chem. 268:21862-21867 (1993)), proteinase-inhibitor complexes, such as .alpha..sub.2 -macroglobulin-proteinase complexes (Ashcom et al., J. Cell Biol. 110:1041-1048 (1990); Moestrup and Gliemann, J. Biol. Chem. 264:15574-15577 (1989)), serpin-proteinase complexes (Orth et al., Proc. Natl. Acad. Sci. USA 89:7422-7426 (1992); Nykj.ae butted.r et al., J. Bio. Chem. 267:14543-14546 (1992); Poller et al, J. Biol. Chem. 270:2841-2845 (1995)), matrix proteins, such as thrombospondin (Mikhailenko et al., J. Biol. Chem. 270:9543-9549 (1995)), apolipoprotein E (apoE)-enriched lipoproteins (Kowal et al, J. Biol. Chem. 265:10771-10779 (1990); Beisiegel et al., Nature 341:162-164 (1989)), hepatic lipase (Kounnas et al., J. Biol. Chem. 270:9307-9312 (1995)) and lipoprotein lipase (Chappell et al., J. Biol. Chem. 268:14168-14175 (1993)).
LRP is expressed in many tissues and is a major apoE receptor in the central nervous system (Rebeck et al., Neuron 11:575-580 (1993)). Genetic data implicate inheritance of the .epsilon.4 allele of apoE as a risk factor in AD (Strittmatter et al., Proc. Natl. Acad. Sci USA 90:1977-1981 (1993); Rebeck et al., Neuron 11:575-580 (1993); Poirier et al., Lancet 342:697-699 (1993); Saunders et al., Neurology 43:1467-1472 (1993)). A possible involvement of LRP in AD is suggested in part by the observation that LRP, as well as apoE and other LRP ligands, decorate senile plaques (Rebeck et al., Ann. Neurol. 37:211-217 (1995)).
The ability of LRP to mediate the cellular catabolism of TFPI, a KPI-containing protein, led to the investigation of the role of LRP in the catabolism of APP.sub.s 770. The present inventors have found that LRP is capable of binding and mediating the internalization and degradation of APP.sub.s 770 as well as its complexes with proteinases.
Because catabolism of APP has been shown to generate the A.beta. peptide, which is believed to be the causative agent of Alzheimer's Disease, there is a need for compositions and methods which reduce the interaction, cellular internalization and subsequent catabolism of APP.